X-ray detectors are used in different areas of medical imaging, for example in computed tomography, angiography or radiography. X-ray photons can be detected, for example, in the energy range from 5 keV to 1 MeV. A preferred energy range is 20 keV to 200 keV, which is advantageous in medical imaging. Apart from X-ray radiation, gamma radiation can also be detected.
The X-ray detector can convert the X-ray radiation into an electrical signal or an electrical pulse with the aid of a converter material. The conversion can take place via direct or indirect converter materials. Preferably, direct converter materials are used, for example CdTe, CZT, and other converter materials comprising Cd, GaAs or Si. The conversion of the energy of the X-ray photons deposited in the converter material can lead to the triggering of electron-hole pairs. A charge or a current can be measured as the signal. Furthermore, for example, amorphous selenium can be used as a photoconductor or a converter material.
In counting X-ray detectors, the electrical pulses are amplified in an integrated circuit or ASIC (Application Specific Integrated Circuit). The amplified pulse is compared in one or more discriminators with an energy threshold. On use, for example, of a one-sided discriminator, the pulse is registered as an event. The number of registered events is a count value. In particular for uses with high photon fluxes, for example, in computed tomography, many bits are involved during the readout of the count values. For each detector element, the number of bits represents the summand of the number of energy thresholds or discriminators and the length of the counter.
Conventionally, the count values of an arrangement of detector elements are read out via shift registers column by column or for the whole arrangement to a serial or parallel output. Alternatively, buses can be used for specifically addressing and reading out individual pixels. Alternatively, detector elements which have a count value of greater than zero can be read out whilst other detector elements with count values equal to zero are not read out. For uses with high photon fluxes, for example, in computed tomography, these methods are not suitable or only to a very limited extent. When shift registers are used for reading out columns or whole arrays of detector elements, a large number of simultaneously switching flip-flops is used. These flip-flops must be operated for a short readout time with a high frequency. The high frequency leads to a high power loss. In addition, the crosstalk can be increased. On use of buses, powerful drivers are needed in order to recharge high parasitic loads of the bus lines rapidly. The use of powerful drivers can lead to a high power loss.
In computed tomography, the X-ray detector is continuously irradiated during a scan or a recording. It is desirable to record the successive images or scans as far as possible without interruption. The readout of the count values should therefore take place during the recording.